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How many photons in the universe? Go on, have a guess

Given that everything started from a singularity, it should not really be a surprise that the number of photons produced by the universe is finite – but somehow it is.

Even more surprising, however, is the news that a team of astrophysicists has succeed in counting them.

In a paper published in the journal Science, researchers led by Marco Ajello from the Clemson College of Science in South Carolina, US, report that the total number of photons produced in the 13.7 billion years since the Big Bang is 4x1084.

To put it another way, that’s 4,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 photons.

To arrive at this figure, Ajello and his colleagues used data from NASA's Fermi Gamma-ray Space Telescope.

The telescope measures gamma rays and their interaction with a cosmic fog interstellar dust and all the ultraviolet, visible and infrared light emitted by stars. Of particular interest to the researchers were gamma ray signals from 739 galaxies containing supermassive black holes, known as blazars.

By measuring the levels of gamma ray photons contained within ultra-high-speed jets produced by the blazars it was possible to estimate not only the density of surrounding cosmic fog at any given place but also to calculate it for any given time in the history of the universe.

“Gamma-ray photons traveling through a fog of starlight have a large probability of being absorbed,” explains Ajello.

“By measuring how many photons have been absorbed, we were able to measure how thick the fog was and also measure, as a function of time, how much light there was in the entire range of wavelengths.”

This range, not including gamma rays, is known as extragalactic background light (EBL) and until recently its density could only be roughly estimated. Thanks to Fermi's Large Area Telescope, calculations are now considerably more precise.

“By using blazars at different distances from us, we measured the total starlight at different time periods,” says co-author Vaidehi Paliya.

“We measured the total starlight of each epoch – one billion years ago, two billion years ago, six billion years ago, and so on – all the way back to when stars were first formed. This allowed us to reconstruct the EBL and determine the star-formation history of the universe in a more effective manner than had been achieved before.”

The result not only provided the most accurate estimation of the EBL to date, but also revealed its cosmic history in unprecedented detail. Co-author Dieter Hartmann calls it a major breakthrough in the field.

“Star formation is a great cosmic cycling and recycling of energy, matter and metals,” he says. “It's the motor of the universe. Without the evolution of stars, we wouldn't have the fundamental elements necessary for the existence of life.”

The other matter that the study makes plain is that despite the mind-bogglingly large number of photons produced since it began the universe is – relatively speaking – rather a dimly light place.

Ajello and colleagues calculate that once the sun and the rest of the Milky Way are discounted, the amount of light that reaches the Earth from the entire rest of the cosmos is equivalent to that produced by a 60-watt bulb seen from about four kilometres away.